Excitation system



2 fsHEETs-sHEE'r 1 REVERSE 2 2 CURRENT RELAY M. A. EDWARDS ETAL EXCITATION SYSTEM Fig].

Oct. 28, 1952 Filed Sept. 13, 1949 Inventors: Martin AElclwar-ds,

Hugh M. Ogle,

Donald I-lGaPr Their Attorney.

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8, 1952 M. A. EDWARDS ETAL EXCITATION SYSTEM 2 Sl-lEETS-Sl-IEET 2 Filed Sept. 13, 1949 Fig. 4.

GENERATOR CURRENT w rink. n mm W @E vAmh A $3 .r mmH e a m M W AMFERE-TURNS Patented Oct. 28, 1952 EXCITATION SYSTEM Martin A. Edwards, Scotia, and Donald E. Garr and Hugh M. Ogle, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application September 13, 1949, Serial No. 115,426

2 Claims.

Our invention relates to excitation systems for dynamoelectric machines and more particularly to the use of such systems in connection with the charging of storage batteries.

In the charging of storage batteries by the use of a dynamoelectric machine as a charging generator it is frequently desirable to maintain a constant charging voltage across a battery under various conditions of battery loading or state of charge. It is further generally desirable to limit the magnitude of the current supplied by such a charging generator to a safe value relative to the design thereof. In the case of a relatively large generator, the excitation therefor is frequently supplied by the use of an auxiliary dynamoelectric machine, such as an amplidyne generator, as an exciter, and control of the characteristics of the charging generator is effected by control of the excitation supplied to the exciting generator. Accordingly to regulate the output voltage of a charging generator in such a system a voltage regulating arrangement responsive to the generated voltage is employed to control excitation supplied to the exciter generator. Similarly, to limit the current output of the charging generator 21. current-limiting arrangement is employed to control excitation supplied to the exciting generator.

It is an object of our invention to provide a new and improved excitation system for dynamoelectric machines which is particularly useful in charging storage batteries but not limited to this particular application.

It is another object of our invention to provide a new and improved current-limiting arrangement in an excitation system for dynamoelectric machines.

It is a further object of our invention to provide in an excitation system for dynamoelectric machines a current-limiting arrangement having a relatively small voltage drop thereacross and a relatively sharp cut-in point, the latter being easily adjustable over a substantial range of values.

Broadly speaking, our invention provides an excitation system for a main dynamoelectric machine or generator which is arranged to be excited by an auxiliary dynamoelectric machine or exciter. A regulating circuit is arranged to maintain the voltage output of the main generator constant regardless of load thereon, within limitations in the current capacity of the generator. Beyond a predetermined current use is made of a magnetron device in a current-limiting arrangement to reduce the generator voltage out put to reduce correspondingly the generator current output.

Our invention will be better understood from the following description taken in connection with the figures of the accompanying drawing, in which Fig. 1 represents diagrammatically a battery charging arrangement employing an excitation system embodying certain features of our invention; Fig. 2 is a cross-sectional view of a magnetron device; Figs. 3a, 3b, 3c are simplifled diagrams of a portion of Fig. 1 in which certain assumed circuit conditions are shown; and Figs. 4 and 5 are curves representing certain characteristics of the arrangement of Fig. 1.

Referring now to Fig. 1, there is shown a battery charging system in which a dynamoelectric machine I is arranged to be driven by an engine 2 such as a diesel engine, and to supply charging current to a storage battery 3 which is represented as including a plurality of individual cells 4. The system shown may conveniently be employed in connection with diesel-electric locomotives and the like wherein storage batteries may be required for starting and other purposes. In such an arrangement engine 2 may be a relatively large main engine employed to provide motive power for the locomotive and may be operated over a relatively wide range of speeds. Generator I may be belted or otherwise suitably connected to engine 2 and is preferably employed solely for the purpose of charging storage battery 3.

Generator l is provided with a series field exciting winding 5 and a shunt field exciting winding 6. armature terminals of generator I and is arranged to conduct the generator armature current thus providing a portion of the excitation for generator I. The principal portion of such excitation however is supplied through winding 6 by an auxiliary dynamoelectric machine 1 which is employed as an exciter. Exciter 1 is preferably an amplidyne generator of the type described and claimed in U. S. Pat. No. 2,227,992 of Alexanderson et al., assigned to the same assignee as the present invention. Amplidyne is provided with a pair of main armature terminals 8 and 9, a pair of quadrature terminals l0 and l I short-circuited by a conductor 12, and a plurality of field exciting windings l3, M, l5 and I6.

Field exciting winding I3 is employed as a compensating winding for amplidyne 1. One terminal of winding [3 is connected to armature terminal 9 and in efiect the other terminal of winding l3, rather than terminal 9, actually consti- Winding 5 is connected to one of the.

- the negative terminal of battery 3.

tutes one output terminal of amplidyne 1, and is designated in the drawing by numeral 6'. Field exciting winding id is employed as an antihunt winding. One terminal of winding 56 is connected to armature terminal 9 and the other terminal of winding M is connected to an antihunt capacitance 51, which in turn is connected to armature terminal 8, thus connecting the series combination of winding id and capacitance 11 across output terminals 8 and 9 of amplidyne 1. The function of winding H3 is toprovide an anti-hunt effect in the regulating system to be described hereinafter.

Exciter 1 is arranged to be driven by a, motor I8 having an armature 19, a field exciting winding 26 connected in series with armature l6, and a second field exciting winding 2! connected in shunt relation across the series combination of armature l9 and winding 29.

Attention is next directed to the connections between generator I and battery 3. In particular the positive output terminal of generator l is connected through the series windings of a conventional reverse current relay 22 to a switch 23 and thenceto the-positive terminal of battery 3.

through series field exciting winding 5 and the serieswinding of a magnetron device 2d, which will be described in greater detail hereinafter, to

of buses of opposite polarity are established between generator I and battery 3. The portion of the positive bus between generator I and relay 22 is designated in the drawing by numeral and the portion between relay 2'2 and switch 23 by numeral 26. The portion of the negative bus between magnetron 24 and battery 3 is; designated by'numeral 21.

, Motor 161s: connected in series with a switch 26 between buses 26 and 21, the junction point between motor l8 and switch 28being designated by numeral 29. Generator field exciting, windin 6 is. connected to junction point 29 and to output terminal 8 of exciter 1. Field excitingywinding l5 *of'exciter' 1 is connected to output terminal 9 thereof and to bus- 21, An adjustable-resistance "-15 connected in parallel relation with winding l5 to act as a variable shunt to control the exciting strength'of winding, 15. From the foregoing description it will be seen that when switch 28 is closed, motor 28 is efiectively connected between buses 26 and 21', and similarly the series combination of winding 6 and exciter 1 is connected between buses 26 and 21.

It will be helpful in the understanding of our invention to describe briefly the operation of the batterycharging system thus far describedbefore proceeding with the description of. the voltage regulating and current limiting features of the arrangement. 'If it is assumed that switches 23' and 28 are open and that engine 2 is drivinggen- -erator 3' at a substantially constant speed, then battery I3 is disconnected from bus 26 and substantially zero voltage is generated by generator I! since no excitationis'provided therefor.

tendency for battery 3 to circulate a current through generator I. of reverse current relay 22 the connection between buses 25 and 26 is interrupted. Therefore,

However due tothe action the voltage'of battery 3 exists between buses 26 The negative terminal of generator I is connected Thus, a pair 4 and 21, but no voltage exists between buses 25 and 21.

If switch 28 is closed the voltage of battery 3 is impressed on motor l8 causing rotation of motor I8 and exciter 1, which is driven thereby. The voltage of battery 3 is likewise impressed on the series combination of field exciting winding 6, exciter 1 and field exciting winding l5-causing 3. current to flow therethrough. The flow of current through winding 6 causes excitation to be provided for generator I causing a voltage to be generated thereby and to be applied between buses '25 and 2-1.

The'fiow-of current through winding l5 likewise causes excitation to be provided for exciter 1 and causes'a voltage to be generated thereby across output terminals 8 and 9. The polarity of the voltage generated by exciter 1 due to excitation by winding I5 is such that the potential of terminal 9' is positive with respect to the potential of terminal 8 and ineffect the voltage generated by exciter 1 is caused to act in series with the voltage of battery 3 between buses-26 and 21, thereby providing the sum of these voltages to be employed in circulating an exciting current through field exciting winding 6. By use of the arrangement shown a substantial portion of the excitation of winding 6 is obtained from the voltage between buses 26 and 21, thereby reducing the power requirements of exciter 1. This is particularly the case since the speed of generator I may vary over a considerable speed range, with an accompanying variation in the excitation requirements thereof. v

With the establishment of a voltage of sufiicient magnitude by generator I across buses 25 and 21 action'of reverse current relay 22, which is provided with a shunt connection 3| to bus 21,. is effected in a conventional manner to complete the connection between buses 25 and 26. Once the connection between buses 25 and 26 is completed the eifect of relay 22 may-be'ignored in the operation of the battery charging system. It will be understood that reverse current relay 22 per so does not constitute a part of our invention.

Without suitable regulation, the magnitude of current circulated through winding 6 is unregulated and suitable characteristics of generator I are not obtained for charging battery 3. To regulate the magnitude of the voltage of generator I by controlling the excitation thereof, use "is made of field excitin winding is and an associated regulating circuit to control the excitation and output voltage of exciter 1. A gaseous electron discharge device32, such as-a neon tube, having a pair of electrodes33- and 34,-is employed as a reference voltage device. Electrode 33 is connected to bus 25 and electrode 34 is connected through a current limiting resistance 35 to bus 21. As is wellknown, when av gaseous discharge device is caused to-break down and conduct-current from one electrode to the other, the'voltage drop through the gaseous discharge between the electrodes is substantially constant over a considerable range of conducted current. Thus for a considerable variation in voltage between buses 25 and 21 the-voltage drop betweenelectrodes 33 and 34 is. substantially constant and maybe conveniently employed as a voltage reference.

An electron discharge. device 36 is employed to makeuse of the reference voltage providedby gaseous discharge device 32. Electron discharge device 36 ispreferably of the beam-power pentode type. having acathode-31, ananode 38, a

suppressor electrode 33, a screen electrode 40, and a control electrode 4|, the latter being employed to control the flow of current frOm anode 38 to cathode 31. Anode 38 is connected to bus through field exciting winding |6 of exciter 1.

Suppressor 39 is connected to cathode 31 and screen 40 is connected to bus 25. Control electrode 4| is connected in series relation with current limiting resistance 42 to the junction point, designated by numeral 43, between electrode 34 and resistance 35.

Magnetron 24, which is employed in connection with electron discharge device 36, comprises a pair of electron discharge devices 44 and 45, a potential winding 46, and a current winding 41. A physical structure which may conveniently be employed for magnetron 24 is shown in Fig. 2, wherein it will be seen that electron discharge devices 44 and 45 are co-axially positioned at the approximate axial center of a cylindrical field exciting coil 48 made up or" windings 46 and 41. Coil 48 is preferably surrounded by a cylindrical shell 49 of a magnetic material such as iron.

It is well known that electrons flowing from a cathode to an anode in an electron discharge device may be deflected from their normal paths by action of a magnetic field and it is upon such action that the operation of a magnetron is based. Thus it will be seen in the arrangement shown in Fig. 2 that a magnetic field created 2 by flow of current through windings 46 and 41 may be made to act upon electrons flowing in electron discharge devices 44 and 45. The manner in which use is made of magnetron 24 as a current limiting device will be more fully described hereinafter.

Referring again to Fig. 1, each of electron discharge devices 44 and 45 comprises a pair of diode sections. One diode section of discharge device 44 comprises a cathode 56 and an anode 5|, while the other diode section comprises a cathode 52 and an anode 53. Similarly, the diode sections of discharge device 45 comprise a cathode 54 and an anode 55, and a cathode 56 and an anode 51, respectively.

Electron discharge devices 36, 44 and 45 have, in addition to the elements previously mentioned, cathode-heating elements 58, 59, and 63, respectively, which for the sake of clarity are shown in Fig 1 in relation to the circuit connections thereto. In particular heating elements 58-60 may be conveniently connected in series relationship between buses 26 and 21 so that energy is supplied to the heating elements whenever buses 26 and 21 are energized by closing switch 23. In this manner the cathode elements of electron discharge devices 36, 44 and 45 are heated, the devices thereby being rendered operative whenever voltage exists between buses 26 and 21 and regardless of whether voltage exists between buses 25 and 21.

The diode sections of electron discharge devices 44 and 45 are connected in series relation. Anode 5| is connected to bus 25, cathode 56 is connected to anode 53, cathode 52 is connected to anode 51, and cathode 56 is connected to anode 55. Cathode 54 is connected to one terminal of a rheostat 6| having a movable tap 62, and the other terminal of rheostat 6| is connected to a fixed resistance 63, which in turn is connected to bus 21. Cathode 31 of electron discharge device 36 is connected to movable tap 62 of rheostat 6l.

One terminal of potential winding 46 of magnetron 24 is connected to bus 25. The other terminal of winding 46 is connected to an adjustable resistance 64 having a movable tap 65. Resistance 64 is connected to a fixed resistance 66 which is also connected to bus 21. Winding 41 is connected in series with generator and is arranged to conduct the current supplied thereby.

In the foregoing description mention has already been made of the manner in which charging generator I is caused to generate a voltage which, without a regulating action, is uncontrolled and hence is unsuitable for use in charging storage battery 3. The regulation of the voltage of generator or the voltage between buses 25 and 21 which is substantially the same as the voltage of generator I, is eliected by control of current fiow through field exciting winding l6 through action of electron discharge device 36.

For the purpose of charging storage battery 3 it is desirable to maintain this voltage at a substantially constant value. A reference voltage is obtained by the use of gaseous discharge device 32 in which a voltage of substantially constant magnitude is established between electrodes 33 and 34. By connecting control electrode 4| of electron discharge device 36 to electrode 34 at junction 43, the potential of control electrode 4| is maintained substantially constant with respect to the potential of bus 25.

The diode elements of electron discharge devices 44 and 45 are operated in such a manner that the series combination of the four diode elements behaves as a resistance element. Thus the series combination of electron discharge devices 44 and 45, rheostat El, and resistance 63, connected between buses 25 and 21, constitutes in effect a voltage divider. Since cathode 31 of electron discharge device 36 is connected to movable tap 62, of rheostat 6|, the potential of cathode 31 is adjustable with respect to the potential of bus 25. Since the potential of anode 38 is positive with respect to the potential of cathode 31 a current tends to flow from anode 38 to cathode 31 the magnitude of such current depending upon the potential of control electrode 4| with respect to cathode 31.

The current flowing through electron discharge device 36 also flows through field exciting Winding I6, the direction of such flow being indicated by an arrow 61. The direction of flow of cur-- rent through winding 16 is such that the excitation provided thereby for exciter 1 opposes the excitation provided by winding I5, the flow of current through winding l5 being indicated by an arrow 68. In other Words, an increase of current flowing through winding l6 tends to cause the total excitation provided for exciter 1 and the voltage generated thereby to be decreased.

The regulating action which is obtained through the use of electron discharge device 36 may best be understood by a consideration of certain assumed conditions. If it is assumed that the generated voltage between buses 25 and 21 tends to decrease, the potential of cathode 31 with respect to bus 25 tends to decrease, while the potential of control electrode 4| tends to remain constant, and, therefore, the potential of cathode 31 becomes more positive with respect to potential of control electrode 4|. Thus the current conducted by electron discharge device 36 and field exciting Windin I 6 tends to decrease, causing an increase in the net excitation of exciter 1 and an accompanying increase in the voltage generated thereby. The increase in the voltage of exciter tends to increase the I acre-.072

Thus for theassum'ed decrease of voltage of generator I an increase in excitation therefor and -'an'accompanying increase in voltage generated thereby-are provided by the regulating action-de- "scribed. 'The value of regulated voltage may be conveniently adjustedby varying the-setting of adjustable tap 62 of rheostat 6! thereby varying theotential of cathode-31 with respect to the "potential of control electrode'4 I.

To aid further in 'understandingthe regulating action described it 'willbe helpful to consider "examples containing specific-values of voltage in the-' regulating circuit. 'The examples described hereinafter are shownin connection with simplified-schematic diagrams in Figs. 3a, 3b, and 3c. The same numerals are used in Fig. 3 as iriFig. 1

for equivalent elements, except that the series --combination of' the diodes of electron discharge device's '44 and 45 are represented in Fig. 3 by a single resistance element having numeral 44", "and resistances GI and 63 bya single resistance element having numeral 66. Referring to Fig.

3a, if it is'assumed that the normal voltage-of storage battery 3 is approximately 75 volts which 'is-avoltage cornmonly'employed in diesel-electric locomotives, then a voltage ofapproximatelytl) "v'o'lt's' may be required for charging purposes betweenbuses 25 and 2'1. "mately' 65 volts may conveniently be maintained acrossgaseous d'is'charge-devic'e'32 causing control "electrode-4| to be- 65 volts negative with respect r to bus 25. 'The setting of adjustable tap 62' may "be made such that the potential of cathode "37 "is approximately 64 volts negative with respect -to bus 25, andtherefore control electrode 4| is A voltage of approxio'ne' volt negative with respect to cathode 37!.

Rl'eferrin'gnow to Fig. 329, if thevoltage' between bus'es25 and'2'! decreases to 75 vo1ts.'for'exarnp1'e,sthe potential of cathode -31 tends to de crease'to approximately 60 volts negative with respect to'bus 25, while thepotential'o'f control :electrode 4| remainsat the previous value'of 65 volts negative with respect to bus 25. Therefore l control electrode 4| becomes 5 volts'negative'with respect to cathode 31, While previously electrode 4| was only 1 volt'negative with respect to cathode 31. Under this condition the current conducted by electron dischargedevice Stand: winding- I6 tends to be decreased causingthe excitation of exciter landthe voltage ofgenerator I to be increased.

.It will be understood-that the regulatingaction provided by electron discharge device 3i'5 is obtained by variation of the potential of cathode 31 with'resp'ectto the potential or control electrode '41, the latter being substantially constant in po- 1 tential 'due" to thesubstantially constant poten- "tial drop across gaseous discharge device 32. The

desired voltage characteristic of generator I is shown graphically'by curve 69in Fig. 4, wherein "the output voltage'of generator I is plotted as a function of the current suppliedflby generator in charging-storagebattery3. It will be noted -that the generated voltage is maintained substantially constantover a considerable range of current values. However, whena value of current is' reached, which for the particular'gnerator exn'ployedis relatively high,it isdesirable' to reduce the generated voltage sharply to limit the generator output current; as indicated by curve '89.

{The current limiting action of "m'agnetron'24 is obtained by varying the e'fiectiveresistance of 7'5 "noted from curve It that the diode resistanceiis substantially constant over a considerable range of a'mpere-turns' and that-a sharp increase in resistance is encountered at a relatively large value of ampere-turns corresponding to' a r'ela- 'tively-larg'e generator current.

Such" a characteristic is obtained through the useof a magnetron by action-of a magnetic field on'electrons traveling from the cathode tothe anode inan electron discharge device. Under action ofinagneticfiux the electrons; whichnorrnally travel ina straight path, are deviated spirallvfrom their normalcour-seand at a relativelycriti'cal point the spiral paths are suffici'ently sharp-to cause a large proportion'of the electrons to return to the cathode rather than reach the plate. 'A further increasein magnetic flux-prevents additional" electrons from reachingthe'plate and the current conducted by the discharge device diminishes greatly. The efiect of such an increase'in current'may be-conveniently considered as an increase in effective resistance of an electron discharge devic and is so considered in the-present discussion.

Windings 4'6 and 4''! of magnetron 24 are provided-with a fixed number of turns. -A substantially constant number of ampere turns is providedby'winding46 for a given setting of resistance while the number ampere-turnsprovided by winding 41 is proportional to the current supplied by generator 1. The flow of current'through winding 46 is such that the ampere-turns provided thereby are inopposition-to the ampereturns provided by-windi-ng 41. The function of winding 46 is to provide a convenient adjustment of thepoint'orcut-in of thecurr'entlim'itaction of 'magnetron24. When the net value of ampereturns of windings 46 and 4'! causes the magnetic flux provided in the vicinity'of discharge devices 54 and 45 tor'each the cut-invalue, the potential dropacrossdevices 44 and-45 increasessub- 1 s'tantiallycausing the regulated voltage between buses 2'5'and' 2T'tends'to be decreased to reduce the current supplied by generator I.

'Referring'to Fig, 30, there is sh'owna circuit arrangementsimilar' to Figs. 3a and 327 wherein it is assumed that the'currentioutput of generator l is relativelylarge. 'Accordingly; theefiecti've' 'r'esis'ta'nce'o'f electron discharg devices 44 and 45" represented"by resistance element .44 is ubstantiallyincreased beyond the normal value there-cf as represented inthe examples of Figs. 3a

and 3b. With an increase in the value of resistance 44' the disp'ositi'on'of voltage between buses '2 5 and. 21 across resistances l4" and. 66 is "modifl'ed. More particularly; with an increase in the value of resistance t4 th voltage 'drop' thereacro'ssis'" proportionally greater with respec't'to the voltagebetween buses 25* and 21 than would normally be the case.

In Fig.3c1itis assumed that'the voltage across 'resistance 44" is 70 volts and thus thepotential of cathode 37 is 70 volts negative with respect to the potential of bus 25. However, as in the previous examples, th potential of control electrode 4| remains substantially constant at a value of 65 volts negative with respect to bus 25, thereby making the potential of control electrode 4| five volts positive with respect to the cathode 3?. Under this condition the flow of current through electron discharge device 36 and winding I6 is increased causing a net decrease in the excitation of exciter I and a corresponding decrease in the voltage of generator I, which is represented as being '75 volts. A decrease in the voltage of generator I results in a decrease in the current supplied by generator I. The voltage of generator I is reduced sufficiently to limit the current thereof to a predetermined maximum valu as indicated by curve 69 in Fig. 4.

It will be understood that hte current limiting action which is obtained through the use of magnetron 24 is in effect similar to the voltage regulating action previously described, in that the current flowing in electron discharge devic 35 and winding I6 is controlled by varying the potential of cathode 3'! with respect to the substantially constant potential of control electrode 4|.

It will be noted that certain advantages are obtainable through the use of the current limiting arrangement of our invention in excitation systems of the type herein described. In particular, by th use of a magnetron device as a current responsive element, the voltage drop attributable to such an element may be made relatively low to minimize power losses therein. Furthermore in the magnetron device employed in the embodiment of our invention herein described, the fiux provided by potential winding 46 is arranged to oppose the flux provided by current winding 41 in response to the current out put of generator I, causing a feed-back action due to changes of the voltage of generator I which is impressed on winding 46. The preence of such feed-back tends to provide a relatively sharp cut-in of the current limiting action of the regulating system.

The excitation characteristics obtainable through the use of our invention are particularly suitable for battery charging applications, although our invention is not limited to such applications. The absence of moving contacts and other mechanical devices, with the exception of a conventional reverse current relay, is particularly advantageous in reducing maintenance and troublesome operation to a minimum. In addition in battery charging application all necessary operating and control voltages may be obtained from the battery itself without the use of auxiliary sources.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An excitation system tor a dynamoelectric machine including a field exciting winding, dynamoelectric means arranged to energize said field exciting winding, control means including an electron discharge device having a cathode, an anode and a control electrode to vary the energization provided by said dynamoelectric means, means to maintain said control electrode at a potential substantially constant with re spect to the potential of said anode, means responsive to the voltage output of said dynamoelectric machine to vary the potential of said cathode with respect to the potential of said control electrode, means including a magnetron device responsive to the current output of said dynamoelectric machine to vary the potential of said cathode with respect to the potential of said control electrode, said control means being arranged to maintain the polarity of said voltage output fixed relative to the output terminals of said dynamoelectric machine and in response to variation of said potential of said cathode to regulate said voltage output at a substantially constant value for values of said current output below a predetermined value and to decrease said voltage output for values of said current output above said predetermined value to limit said current ouput to a predetermined maximum value, said maximum value being adjustable over a substantial range.

2. An excitation system for a dynamoelectric machine including a field exciting winding; an auxiliary dynamoelectric machine connected to energize said field exciting winding; means for controlling the current in said winding in response to the voltage output of said dynamoelectric machine, said means comprising an electric motor to drive said auxiliary dynamoelectric machine and connected to be supplied with an output voltage from said dynamoelectric machine; a control field winding on said auxiliary dynamoelectric machine; and means controllin current in said control field winding to limit the current output of said dynamoelectric machine to a predetermined value, said last-mentioned means including a magnetron having a current winding connected in the output circuit of said dynamoelectric machine.

MARTIN A. EDWARDS. DONALD E. GARR. HUGH M. OGLE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,875,043 Leece et a1 Aug. 30, 1932 2,334,179 Edwards et a1 Nov. 16, 1943 2,452,611 Stratton Nov. 2, 1948 

